The natural biology of the complex group of murine C-type RNA viruses is being studied using virological, genetic and biochemical approaches. Continuing studies of the genetic transmission of both ecotropic and xenotropic virus classes have led to the chromosomal mapping of several additional virus-inducing loci. Studies of the recombinant MCF viruses in relation to murine leukemogenesis have delineated certain biological and biochemical correlates of the ability of a given MCF isolate to induce or accelerate leukemia, leading to the recognition of target tissue host range as the major determinant of leukemogenicity. Genetic control of sensitivity to MCF virus infection is under investigation in vitro and in vivo.